Gut bacteria shape the newborn immune system with neurotransmitters

Gut bacteria-produced serotonin encourages immune tolerance in infancy.

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Researchers at Weill Cornell Medicine found that particular bacteria start living in the gut soon after a baby is born. The neurotransmitter serotonin, produced by these bacteria, instructs the immune cells in the gut. As the baby ages, this helps prevent allergic reactions to food and the bacteria themselves.

The study’s senior author is Dr. Melody Zeng, an assistant professor of immunology at the Gale and Ira Drukier Institute for Children’s Research and the Department of Pediatrics at Weill Cornell Medicine explained, “The gut is now known as the second human brain as it makes over 90 percent of the neurotransmitters in the human body. While neurotransmitters such as serotonin are best known for their roles in brain health, receptors for neurotransmitters are located throughout the human body.”

Researchers discovered that the stomachs of newborn mice have larger concentrations of neurotransmitters, such as serotonin, than those of adult mice. Dr. Zeng explained that while in adults, a type of gut cell called enterochromaffin cells produces neurotransmitters, in newborns, most of the serotonin is made by bacteria that are more common in their guts.

This discovery was also confirmed in human babies through a stool biobank established by Dr. Zeng’s lab in collaboration with the Neonatal Intensive Care Unit at NewYork-Presbyterian Alexandra Cohen Hospital for Women and Newborns. Samples were collected with parental consent and made anonymous.

The study suggests that gut bacteria provide these necessary chemicals during early development before a baby’s gut can produce neurotransmitters.

The study’s lead author, Dr. Katherine Sanidad, a postdoctoral associate in pediatrics at Weill Cornell Medicine, clarified that gut bacteria in young mice produce serotonin independently and inhibit the monoamine oxidase enzyme, which usually breaks down serotonin. This keeps the gut’s serotonin levels elevated.

These high serotonin levels help balance the immune system by increasing Tregs, preventing the immune system from overreacting and attacking gut bacteria or food. Dr. Sanidad stressed that these serotonin-producing bacteria are essential to the baby’s stomach because they regulate the immune system.

Dr. Zeng emphasized the significance of early colonization with the proper kind of beneficial bacteria in the stomach after birth. Babies in developed countries, who often receive antibiotics early, have less exposure to different microbes and may eat less healthy diets. This can reduce the amount of bacteria that produce serotonin in their guts.

These infants may, therefore, have fewer Tregs, which may result in food allergies or immunological responses directed against their own gut flora. This could be the reason behind the rise in childhood food allergies, particularly in wealthy nations.

Dr. Zeng explained, “Babies would know that foods like peanuts and eggs are safe, and they wouldn’t attack them.” This could also affect the development of autoimmune diseases, where the immune system mistakenly attacks the body’s cells later in life.

The team’s next goal is to investigate the amount of serotonin and other compounds produced by bacteria in human baby feces samples. These substances may instruct the immune system on how to fend off illnesses like cancer, infections, and allergies in the future.

The research shows how important it is for certain chemicals made by gut bacteria to help shape the immune system of newborns. Understanding this process could lead to potential interventions to mitigate the risk of autoimmune diseases and allergies later in life.

Journal reference:

  1. KATHERINE Z. SANIDAD, STEPHANIE L. RAGER et al., Gut bacteria–derived serotonin promotes immune tolerance in early life. Science Immunology. DOI: 10.1126/sciimmunol.adj4775.

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